1. Field of the Invention
The present invention relates to an evaporative fuel control system in an internal combustion engine, comprising a canister for adsorbing an evaporative fuel from a fuel tank, a purge control valve provided between an intake system of the engine and the canister, a purge control means for controlling the operation of the purge control valve to control the flow rate of a purge gas from the canister to the intake system, an exhaust gas concentration sensor provided in an exhaust system of the engine, an air-fuel ratio correcting factor setting means for determining an air-fuel ratio correcting factor in accordance with a detection value detected by the exhaust gas concentration sensor, and an air-fuel ratio control means for controlling the air-fuel ratio of an air-fuel mixture supplied to the engine by use of the determined air-fuel ratio correcting factor.
2. Description of the Prior Art
An evaporative fuel control system is conventionally known, for example, from Japanese Patent Application Laid-open No. 45422/88.
When the purging from the canister to the intake system is being carried out, the enrichment of the air-fuel ratio occurs. However, if a lower limit value of the air-fuel ratio is set constant, irrespective of the enrichment of the air-fuel ratio, the air-fuel ratio of the air-fuel mixture supplied to the engine is deviated from a target value because an air-fuel ratio correcting factor is maintained at the lower limit value. Thus, the exhaust gas purifying performance is reduced due to an over-enrichment. There is also a known system in which a problem produced in a fuel supply system (including a fuel pump, a pressure regulator, a fuel injection valve and the like) is detected, or the flow rate of the purge gas is reduced when the air-fuel ratio correcting factor, determined depending upon the detection value detected by the exhaust gas concentration sensor, becomes equal to or less than the lower limit value. However, if the lower limit value is set constant as described above, a misdetection may be caused during execution of the purging from the canister to the intake system, and a canister break-through may be produced due to a reduction in flow rate of the purge gas, in some cases. Particularly, due to an increase in size of the canister and an increase in flow rate of the purge gas, the air-fuel ratio correcting factor is liable to be decreased down to the lower limit value or less, with an attendant possibility of a misdetection and a canister break-through, immediately after the start of the purging from the canister, while remaining in an idle operation for a long time as well as immediately after the long idle operation time is stopped.
Therefore, in the known system (described in Japanese Patent Application Laid-open No. 45442/88), when the purging from the canister to the intake system is being carried out, the lower limit value of the air-fuel ratio correcting factor is always decreased. However, the degree of enrichment of the air-fuel ratio is varied depending upon the concentration of the purge gas, whereas the concentration of the purge gas is varied depending upon the operational state of the engine. In the system in which the lower limit value is always decreased during execution of the purge as described above, there is a problem that when the concentration of the purge gas is low, the lower limit value may be decreased, resulting in a reduced responsiveness of the control of the air-fuel ratio into a target air-fuel ratio.